Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, lap-top computers and even electric cars, efforts have been increasingly made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention, and among them, the development of rechargeable secondary batteries has been the focus of particular interest. Recently, in the development of such batteries, research and development for new electrode and battery design is being done to improve a capacity density and specific energy.
Among currently available secondary batteries, lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and even higher energy densities than traditional batteries using an aqueous electrolyte solution such as Ni-MH, Ni—Cd, and PbSO4 batteries and the like.
Generally, a lithium secondary battery is fabricated by making an anode and a cathode using a material capable of intercalating and deintercalating lithium ions, and filling an organic electrolyte solution or a polymer electrolyte solution in between the cathode and the anode, and produces electrical energy by an oxidation/reduction reaction when the lithium ions intercalate and deintercalate on the cathode and the anode.
Currently, carbon-based materials are primarily being used as an electrode active material for an anode of a lithium secondary battery. Among the carbon-based materials, graphite has a theoretical capacity of about 372 mAh/g, and an actual realizable capacity of current commercially available graphite is from about 350 to about 360 mAh/g. However, the capacity of the carbon-based material such as graphite is insufficient for a lithium secondary battery requiring a high capacity electrode active material.
To meet the demand, another electrode active material is a metal such as silicon (Si) and tin (Sn) that exhibits a higher charge/discharge capacity than a carbon-based material and is electrochemically alloyable with lithium. However, a metal-based electrode active material experiences cracking and pulverization due to a large volume change involved in lithium charging/discharging, and as a consequence, a secondary battery using a metal-based electrode active material has drawbacks of a drastic capacity drop and a short cycle life during charging/discharging cycles.
Accordingly, attempts have been made to use oxide of a metal, for example, Si and Sn, as an electrode active material, to alleviate cracking and pulverization of an electrode active material caused by the use of a metal-based electrode active material. In the case of oxide of a metal such as Si and Sn, due to uniform distribution of nanoscale metal elements in a carbon substrate, cracking and pulverization caused by a metal-based electrode active material may be effectively controlled, but there is a problem with reduced initial efficiency by an initial irreversible reaction between metal oxide and lithium.
To uniformly distribute a metal-based active material at a nanoscale level, studies are recently being made to grow a metal nanowire on a surface of a carbon-based material. However, cycle efficiency reduces by a continuous side reaction of a surface of a metal nanowire with an electrolyte solution, and due to its by-product, a phenomenon occurs in which resistance increases and an electrode thickness increases.